Autophagy is an evolutionarily conserved catabolic degradation cellular process in which misfolded proteins or damaged organelles are first sequestered by a double-membrane vesicle, called autophagosomes. Autophagosomes are then fused with lysosomes to digest and recycle the contents to maintain cellular homeostasis. Autophagy can also be markedly induced by a wide variety of stresses, e.g. nutrient starvation, infection, and aging, for cell survival. Dysfunctional autophagy has been associated with wide ranges of human diseases, such as, e.g., cancer, neurodegenerative diseases, heart disease, diabetes, and bacterial infection (Wirawan et al., 2012; Yang and Klionsky, 2010).
Autophagyis a double-edged sword for many cellular processes, depending upon the genetic background and microenvironment. For example, autophagy can act as a tumor suppressor by preventing oncogenic protein substrates, toxic misfolded proteins, damaged organelles, and reactive oxygen species (ROS) from accumulating to cause genome instability and cancer initiation (Mathew et al., 2009; Yue et al., 2003). On the other hand, higher basal autophagic activity detected in established tumor cells functions to promote the survival and growth of tumors by maintaining energy production under increased metabolic consumption and a hypoxic microenvironment, thereby enabling tumors to escape chemotherapy and/or radiation (Amaravadi et al., 2007; Lock et al., 2011; Lum et al., 2005; Yang et al., 2011). Therefore, dissecting the molecular mechanisms in regulating autophagy and identifying specific autophagy inhibitors or inducers suitable for clinical application are necessary for specifically targeting autophagy to fight human disease.
A large number of chemicals have been found to either promote or inhibit autophagy. Some of these compounds have been widely used to dissect the mechanisms underlying autophagy (Baek et al., 2012). Popular autophagy inducers include mTOR kinase inhibitors, e.g., rapamycin and torin 1 (Hanson et al., 2013), and chemicals inhibiting inositol monophosphatase, e.g., lithium and carbamazepine (Hidvegi et al., 2010). Notably, rapamycin is an immunosuppressant and has recently been used as an anticancer agent (Ravikumar et al., 2004). Lithium has also been used to treat Huntington's disease and other related neurodegenerative disorders (Sarkar et al., 2005). Commonly used autophagy inhibitors include chloroquine (CQ), 3-methyladenine, wortmannin, and bafilomycin (BAF) (Baek et al., 2012; Rote and Rechsteiner, 1983; Seglen and Gordon, 1982; Wu et al., 2013). Since established tumor cells normally activate autophagy to escape chemotherapy and/or radiation (Yang et al., 2011), numerous preclinical studies found that inhibition of autophagy by CQ restored chemosensitivity and promoted tumor cell death by diverse anticancer therapies (Kimura et al., 2013). Although CQ offers great promise for cancer therapy, CQ induces ocular toxicities and damages the renal system, and it is uncertain whether the tolerated doses of HCQ or CQ can be reached in human tumors to effectively inhibit autophagy (Kimura et al., 2013). Moreover, most of the available autophagy inhibitors, like HCQ or CQ, also lack either specificity, potency, or antitumor activity (Janku et al., 2011). Thus, potent and specific inhibitors of autophagy are needed in order to provide a novel and powerful approach for future cancer therapy.
Recently, many new autophagy chemical modulators have been identified either by screens based on clearance of aggregates of mutant a-synuclein in cells or by image-based screens with GFP-LC3 transfected cells. Although these small chemicals are useful pharmacological tools to study autophagy and are potential therapeutic drugs for autophagy-related diseases, many of these compounds still lack either specificity or potency, or both (Rubinsztein et al., 2012). Therefore, the search for specific and potent autophagy chemical modulators must continue in order to gain deep insight into autophagy and provide potential therapeutic drugs.
By screening a Chembridge library (ChemBridge Corporation, San Diego) that contains around 10,000 drug-like or lead-like small chemicals, vacuolin-1 was originally found to induce homotypic fusion of endosomes or lysosomes, thereby forming large vacuoles. Yet, it does not alter other cell structures and membrane trafficking functions (Cerny et al., 2004; Huynh and Andrews, 2005; Shaik et al., 2009). It remains controversial whether vacuolin-1 blocks the Ca2+-dependent exocytosis of lysosomes.